Matter, Mass, Weight and Physical Properties

1. Introduction

Physics is the branch of natural science that studies matter, energy, motion, forces and the fundamental laws governing the physical universe. Mechanics is a major branch of physics concerned with the motion of objects and the forces that affect their motion.

An understanding of mechanics begins with the concepts of matter, mass, weight and physical properties. These concepts provide the foundation for studying motion, force, momentum, energy and fluid mechanics.

2. Matter: Meaning and Characteristics

Matter is anything that has mass and occupies space. Air, water, soil, metals, plants, animals and human beings are examples of matter.

Matter is composed of atoms and molecules. These particles are continuously in motion, although their movement varies according to the physical state and temperature of the substance.

2.1 General characteristics of matter

The main characteristics of matter include:

1. Matter has mass.
2. Matter occupies space and therefore has volume.
3. Matter is composed of particles such as atoms, molecules and ions.
4. Particles of matter are in continuous motion.
5. Particles attract one another through intermolecular or interatomic forces.
6. Matter can change its physical state under appropriate conditions.
7. Matter can undergo physical and chemical changes.

2.2 States of matter

The three familiar states of matter are solid, liquid and gas. Plasma is commonly recognised as the fourth state, while additional states occur under specialised conditions.

Solids: Particles are closely packed and generally vibrate around fixed positions. Solids usually have a definite shape and volume.

Liquids: Particles remain relatively close but can move past one another. Liquids have a definite volume but take the shape of their containers.

Gases: Particles move freely and are widely separated. Gases expand to fill their containers and are highly compressible.

Plasma: Plasma consists of ionised gas containing free electrons and positive ions. It occurs in stars, lightning, fluorescent lamps and certain industrial applications.

2.3 Changes of state

Matter can change from one physical state to another through the absorption or release of energy.

Melting and vaporisation generally require energy input, whereas freezing and condensation release energy.

A change of physical state does not necessarily change the chemical composition of a substance. For example, melting ice produces liquid water, both of which are H₂O.

3. Physical and Chemical Properties of Matter

Properties describe the characteristics of a substance and help distinguish one material from another.

3.1 Physical properties

Physical properties can be observed or measured without changing the chemical identity of a substance.

Examples include:

• Mass
• Volume
• Density
• Colour
• Temperature
• Melting point
• Boiling point
• Hardness
• Solubility
• Electrical conductivity
• Thermal conductivity
• Viscosity
• Elasticity

3.2 Chemical properties

Chemical properties describe how a substance behaves when it undergoes a chemical reaction or changes its chemical composition.

Examples include:

• Flammability
• Reactivity with acids
• Oxidation tendency
• Corrosion behaviour
• Chemical stability
• Toxicity in chemical interactions

Important distinction: Melting ice is a physical change because the substance remains water. Burning wood is a chemical change because new substances are formed.

4. Classification of Physical Properties

Physical properties can be classified into several categories.

4.1 Intensive and extensive properties

Intensive properties do not depend on the quantity of matter present.

Examples:

• Temperature
• Density
• Pressure
• Melting point
• Boiling point
• Refractive index

Extensive properties depend on the amount of matter present.

Examples:

• Mass
• Volume
• Total internal energy
• Total electrical charge

For example, cutting a metal block into two pieces reduces the mass and volume of each piece, but the density of the material remains approximately unchanged under the same conditions.

4.2 Scalar and vector quantities

Physical quantities are also classified according to whether direction is required to describe them.

Scalar quantities have magnitude only.

Examples include mass, time, temperature, distance, speed, energy and volume.

Vector quantities have both magnitude and direction.

Examples include displacement, velocity, acceleration, force and momentum.

This distinction is essential in mechanics because vector quantities must be combined according to vector rules.

5. Mass

Mass is a fundamental physical property that measures the inertia of an object, meaning its resistance to a change in its state of motion.

The SI unit of mass is the kilogram (kg).

Mass is a scalar quantity.

5.1 Characteristics of mass

1. Mass measures the inertia of an object.
2. It is independent of the object’s location.
3. It remains essentially unchanged when an object moves from Earth to the Moon.
4. It is measured using a balance.
5. Its SI unit is the kilogram.
6. Mass is an additive property for ordinary physical systems.

For example, an object having a mass of 10 kg on Earth also has a mass of 10 kg on the Moon.

5.2 Inertial mass and gravitational mass

Two related concepts are important:

Inertial mass: A measure of an object’s resistance to acceleration when a force acts upon it.

Gravitational mass: A measure of how strongly an object participates in gravitational interaction.

Experiments show that inertial and gravitational mass are equivalent to extremely high precision, forming an important foundation of modern gravitational theory.

6. Weight

Weight is the gravitational force acting on an object.

It depends on the mass of the object and the local gravitational acceleration.

The relationship is:

Weight = mass × gravitational acceleration

[
W=mg
]

Where:

• (W) = weight in newtons (N)
• (m) = mass in kilograms (kg)
• (g) = gravitational acceleration in m/s²

Weight is a vector quantity because it acts in a particular direction, normally towards the centre of the attracting celestial body.

6.1 Example: Calculation of weight

An object has a mass of 10 kg. Calculate its weight on Earth’s surface, taking gravitational acceleration as 9.8 m/s².

Solution:

[
W=mg
]

[
W=10\times9.8=98\ N
]

Therefore, the object’s weight is approximately 98 N.

6.2 Mass and weight on different celestial bodies

The mass of an object remains unchanged when it moves between celestial bodies, but its weight changes because gravitational acceleration varies.

For example, gravitational acceleration on the Moon is approximately one-sixth of that on Earth.

An object with a mass of 60 kg has:

• Mass on Earth: 60 kg
• Mass on Moon: 60 kg
• Weight on Earth: approximately 588 N
• Weight on Moon: approximately 97 N

6.3 Mass versus weight

Examination point: A person’s weight can change on the Moon, but the person’s mass remains the same.

7. Density

Density is the mass contained in a unit volume of a substance.

[
\rho=\frac{m}{V}
]

Where:

• (\rho) = density
• (m) = mass
• (V) = volume

The SI unit of density is kilogram per cubic metre (kg/m³).

The commonly used alternative unit is gram per cubic centimetre (g/cm³).

[
1\ g/cm^3=1000\ kg/m^3
]

7.1 Example: Density calculation

A metal block has a mass of 500 g and a volume of 100 cm³.

[
\rho=\frac{500}{100}=5\ g/cm^3
]

Its density is 5 g/cm³ or 5000 kg/m³.

7.2 Relative density

Relative density, also known as specific gravity, is the ratio of the density of a substance to the density of a reference substance.

For solids and liquids, water is commonly used as the reference substance.

[
\text{Relative Density}=
\frac{\text{Density of Substance}}{\text{Density of Water}}
]

Relative density has no unit because it is a ratio of two quantities with the same dimensions.

If a substance has a density of 800 kg/m³, its relative density with respect to water of density 1000 kg/m³ is:

[
RD=\frac{800}{1000}=0.8
]

A substance with relative density below 1 generally floats in water, provided other relevant conditions are satisfied.

8. Other Important Physical Properties

8.1 Volume

Volume is the amount of three-dimensional space occupied by an object.

The SI unit is cubic metre (m³).

Common units include litre (L), millilitre (mL) and cubic centimetre (cm³).

[
1\ L=1000\ cm^3=10^{-3}\ m^3
]

8.2 Temperature

Temperature is a physical quantity that determines the direction of spontaneous thermal energy transfer between bodies in thermal contact.

The SI unit is kelvin (K).

Common temperature scales include Celsius, Fahrenheit and Kelvin.

[
K=^\circ C+273.15
]

[
^\circ F=\frac{9}{5}(^\circ C)+32
]

8.3 Elasticity

Elasticity is the ability of a material to regain its original shape and size after the deforming force is removed, provided the elastic limit has not been exceeded.

Rubber and steel both exhibit elasticity, although their elastic behaviour differs.

8.4 Plasticity

Plasticity is the ability of a material to undergo permanent deformation without breaking when an appropriate force is applied.

Clay and modelling materials exhibit plastic behaviour.

8.5 Hardness

Hardness is the resistance of a material to scratching, indentation or wear.

Diamond is one of the hardest naturally occurring materials.

8.6 Viscosity

Viscosity is a measure of a fluid’s resistance to flow.

Honey generally has a higher viscosity than water.

8.7 Compressibility

Compressibility describes the extent to which the volume of a substance decreases under applied pressure.

Gases are generally much more compressible than liquids and solids.

8.8 Thermal and electrical conductivity

Thermal conductivity describes a material’s ability to transfer heat.

Electrical conductivity describes its ability to conduct electric current.

Copper and aluminium are widely used as electrical conductors, while rubber and many plastics are electrical insulators.

9. Importance of Matter and Its Properties

Understanding matter and its properties is important in:

1. Civil engineering and construction.
2. Selection of materials for buildings and infrastructure.
3. Water treatment and environmental management.
4. Industrial manufacturing.
5. Medicine and pharmaceutical sciences.
6. Agriculture and soil science.
7. Energy production and storage.
8. Material testing and quality control.

For example, engineers consider density, strength, elasticity, durability and thermal properties when selecting construction materials.

10. Key Examination Distinctions

36 Key Takeaways for Exams

1. Matter is anything that has mass and occupies space.
2. Matter is composed of atoms, molecules or ions.
3. The three common states of matter are solid, liquid and gas.
4. Plasma is an ionised state of matter containing free charged particles.
5. Solids have definite shape and volume.
6. Liquids have definite volume but take the shape of their containers.
7. Gases have neither definite shape nor definite volume.
8. Gases are generally more compressible than liquids and solids.
9. Melting is the conversion of a solid into a liquid.
10. Sublimation is the direct conversion of a solid into a gas.
11. Deposition is the direct conversion of a gas into a solid.
12. Physical properties can be observed without changing chemical identity.
13. Chemical properties describe how substances behave in chemical reactions.
14. Density, temperature and pressure are intensive properties.
15. Mass and volume are extensive properties.
16. Scalar quantities have magnitude only.
17. Vector quantities have both magnitude and direction.
18. Mass measures an object’s inertia.
19. The SI unit of mass is the kilogram.
20. Mass remains essentially unchanged when an object moves between planets.
21. Weight is the gravitational force acting on an object.
22. The SI unit of weight is the newton.
23. Weight is calculated using (W=mg).
24. Earth’s standard gravitational acceleration is approximately 9.81 m/s².
25. The Moon’s gravitational acceleration is approximately one-sixth of Earth’s.
26. Density is mass per unit volume.
27. The SI unit of density is kg/m³.
28. One g/cm³ equals 1000 kg/m³.
29. Relative density is a dimensionless ratio.
30. Water is commonly used as the reference substance for relative density of solids and liquids.
31. The SI unit of temperature is kelvin.
32. Absolute temperature in kelvin is obtained by adding 273.15 to Celsius temperature.
33. Elasticity is the ability to recover original shape after removal of a deforming force.
34. Plasticity involves permanent deformation.
35. Viscosity measures resistance to fluid flow.
36. Material properties are essential in engineering, construction, manufacturing and scientific research.